Re: [MMUSIC] Bundle offer with different ports - where to expect media?

Justin Uberti <juberti@google.com> Tue, 21 May 2013 22:39 UTC

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From: Justin Uberti <juberti@google.com>
Date: Tue, 21 May 2013 15:38:50 -0700
Message-ID: <CAOJ7v-398_MiXLWjexU0Z0xQju3A-zWuQFkWkJSLPA5UEGAu+g@mail.gmail.com>
To: Paul Kyzivat <pkyzivat@alum.mit.edu>
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Cc: mmusic <mmusic@ietf.org>, Christer Holmberg <christer.holmberg@ericsson.com>
Subject: Re: [MMUSIC] Bundle offer with different ports - where to expect media?
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Lots of discussion here; is this an accurate summary of the discussed
behavior?

1) BUNDLE offerers must be able to receive on any port (until they get a
BUNDLE-compatible answer)
2) Once the answer is received, bundling is activated, and BUNDLEs use the
port of the first non-rejected m= line in the offer
2a) at this point, the other ports can be discarded
3) All m= lines offered in a BUNDLE must be present in the answer, unless
they are rejected
3a) no selective unbundling/BUNDLE splitting is permitted


On Tue, May 21, 2013 at 10:57 AM, Paul Kyzivat <pkyzivat@alum.mit.edu>wrote;wrote:

> On 5/21/13 4:45 AM, Emil Ivov wrote:
>
>> Hey Paul,
>>
>> On 20.05.13, 18:09, Paul Kyzivat wrote:
>>
>>> On 5/20/13 10:51 AM, Emil Ivov wrote:
>>>
>>>> Hey Christer,
>>>>
>>>> On 20.05.13, 17:16, Christer Holmberg wrote:
>>>>
>>>>> Hi,
>>>>>
>>>>>  What happens when the offerer knows the answerer has bundle
>>>>>>>> support, sends all m-lines with the same port, then the
>>>>>>>> answerer splits the first line away from the bundle? Would the
>>>>>>>> answerer still send everything to the same port?
>>>>>>>>
>>>>>>>
>>>>>>> We discussed this week,
>>>>>>>
>>>>>>>  Yes, sorry, I didn't follow this closely.
>>>>>>
>>>>>>  and the outcome (at least my read of it :) was that the answerer
>>>>>>> is not allowed to split any m- lines away from the bundle in this
>>>>>>> case. Instead the answerer will have to send a new offer for the
>>>>>>> split, allowing new ports to be negotiated at both ends.
>>>>>>>
>>>>>>
>>>>>> OK, so shouldn't the same thing happen in the case with different
>>>>>> ports?
>>>>>>
>>>>>
>>>>> I suggested that it should never be allowed to split an m- line from
>>>>> a bundle group in an answer, but others had other opinions.
>>>>>
>>>>
>>>> I don't see how we could allow it in one case and disallow it in the
>>>> other. The only difference between the two cases is how informed the
>>>> offerer is about the answerers bundle support capabilities and I don't
>>>> really understand why this would influence the decision to allow
>>>> splitting bundles one way or the other.
>>>>
>>>
>>> It is a different case because the same port *can't* both be used as the
>>> bundle port and a port for an unbundled m-line.
>>>
>>
>> Why not? Obviously the offerer was prepared to demultiplex traffic
>> there. Why wouldn't it be able to continue doing so even if the answerer
>> would prefer to receive it separately?
>>
>
> The point of wanting the one line unbundled is so that it can be
> *received* on a different address/port. Perhaps there is a separate device
> or process that will be supporting that one, that must be reached at its
> own addr/port.
>
> Unless you expect that this would have a common addr/port on the offering
> side and different addr/ports on the answering side. But we investigated
> that approach early in the bundle discussions, and gave it up.
>
>
>  Note that I am not defending such an approach. I'd much prefer that
>> splitting is only allowed when re-offering.
>>
>>  HOWEVER, it would still not help in the case where the 1st m- line is
>>>>> rejected.
>>>>>
>>>>
>>>> Well, how about looking at it this way: the offerer specifies a bundle
>>>> port in the first m=line. This also happens to be the port for the first
>>>> media line but the two are different things and just happen to have the
>>>> same value for reasons related to syntax and convenience.
>>>>
>>>> A bundle supporting answerer should understand this. After receiving the
>>>> offer that answerer has learned the bundle port number. Rejecting the
>>>> first m=line in the answer does not change this.
>>>>
>>>
>>> There are many reasons that an answerer may reject an m-line.
>>> It is *possible* that it is rejecting it because it has a problem with
>>> the address (c=) for the m-line in the offer. If so, then if you insist
>>> on using it as the bundle address, even if the m-line is refused, then
>>> there is no way for the answerer to refuse it.
>>>
>>> (*Why* it would have a problem is an open question. Maybe its IPv6 and
>>> the answerer can't use it, or maybe its an FQDN and it can't be
>>> resolved. I realize this is unlikely. But making the assumption that the
>>> address must be acceptable to the answerer is IMO not a good idea.)
>>>
>>
>> Aha! That's a good point.
>>
>> Still, the thing that I don't quite understand is: if the answerer has a
>> problem with the c= line, how could that problem only apply to the first
>> m= line and not to the entire bundle?
>>
>
> Presumably this would only happen if the c= for were not all the same in
> the offer. Certainly that is possible. When allocating "ports" for each
> m-line one might also end up with different addresses.
>
>
>  Of course the answer would come with the first m=line having a 0 port
>>>> but the offerer would then just learn the bundle port number at the
>>>> first m=line with a non-zero port.
>>>>
>>>
>>> I presume you must have a motivation for wanting to go this way.
>>> Are you thinking this will simplify the implementation?
>>>
>>
>> No. The implementation is going to be slightly more complicated indeed
>> because instead of having one bundle demuxing socket and two regular
>> ones (well ... if rtcp demuxing counts as regular) you would have three
>> of them. I don't think this would be particularly horrifying in terms of
>> complexity though.
>>
>> Still, I think being consistent about the ports where one could get
>> multiplexed data is a good thing, and it could help (humans) when
>> analysing traffic and debugging network issues.
>>
>
> Consistency is good. But isn't it just as consistent to say that the first
> *accepted* m-line defines the bundle addr/port as it is to say that the
> first offered m-line?
>
>         Thanks,
>         Paul
>
>
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